In line with an increase in the price of energy sources due to the depletion of fossil fuels and amplification of interests in environmental pollution, environmentally-friendly alternative energies have become an indispensable element for future life. Thus, research into various power generation techniques using natural energy resources, such as sunlight, wind, and tides, has continuously conducted, and great interests in power storage devices for more efficiently using the energy thus generated have also grown.
In particular, the demand for secondary batteries as an environmentally-friendly alternative energy source has rapidly increased as the technology development and demand for mobile devices have increased. The secondary batteries are recently being used as power sources of devices requiring large power, such as electric vehicles (EVs) or hybrid electric vehicles (HEVs), and the application area has been extended to include uses, such as an auxiliary power source through power grids and the like.
In order for the secondary batteries to be used as the power sources of the devices requiring large power, high energy density, excellent safety, and long cycle life are necessarily required in addition to characteristics of generating large output in a short period of time, for example, the batteries must be used for 10 years or more even under severe conditions in which high-current charge and discharge are repeated in a short period of time.
Lithium metal has been used as an anode of a typical lithium secondary battery. However, since it has been known that a battery short circuit may occur due to the formation of dendrites and there is a risk of explosion due to the short circuit, the lithium metal is being replaced by a carbon-base compound capable of reversibly intercalating and deintercalating lithium ions as well as maintaining structural and electrical properties.
Since the carbon-based compound has a very low discharge voltage of about −3 V with respect to a hydrogen standard electrode potential and exhibits highly reversible charge and discharge behavior due to the uniaxial orientation of a graphene layer, the carbon-based compound exhibits excellent electrode cycle life. Also, since the carbon-based compound may exhibit a potential that is almost similar to pure lithium metal, i.e., the electrode potential of the carbon-based compound is 0 V Li/Li+during lithium (Li)-ion charge, higher energy may be obtained when a battery is formed with an oxide-based cathode.
The anode for a secondary battery may be prepared by a method, in which a single anode active material slurry is prepared by mixing a carbon material as an anode active material 13 with a conductive agent and a binder if necessary, and an electrode current collector 11, such as a copper foil, is then coated with a single layer of the slurry and dried. In this case, a press process is performed during the slurry coating in order to press active material powder to the current collector and obtain a uniform thickness of the electrode (see FIG. 1).
However, during a typical press process of an electrode, surface porosity may decrease as the depression of a surface is intensified in comparison to the inside of an anode active material.
Since such a phenomenon may be further intensified when the thickness of the electrode is high, an electrolyte solution may be difficult to penetrate into the electrode. Accordingly, ion transfer path may not be secured and thus, ions may not be smoothly transferred to cause degradation of battery performance and cycle life.